Microwave food containers are well known. Some such containers include microwave susceptors applied to surfaces that partially or completely surround food intended to be heated in a microwave oven. The susceptors, which may be applied in many patterns, interact with microwave energy in a microwave oven to become hot. The hot susceptors then act either as a radiant heat sources when not in direct contact with food or conductive heat sources when directly contacting food. This, in turn, can augment the internal heating of food through microwave excitation by providing surface browning or crisping mimicking that obtained in traditional radiant ovens.
Microwave susceptors of food containers often are ultra thin films of metals such as aluminum deposited by vacuum evaporation or other means upon a polyester substrate. These polyester/metal susceptors are then bonded to the paper of the container on interior surfaces thereof. The susceptors can be rapidly heated by exposure to microwaves to temperatures of 400 degrees Fahrenheit or more to provide radiant or conductive heating. Other materials have been used as susceptors including steel, copper, and Inconel® alloy. A defect such as, for instance, a cut, crack, gap, or poorly deposited area of metal in a susceptor can occur during manufacture or fabrication. Such defects only become apparent when a consumer attempts to cook a microwave dish and can manifest themselves in various ways including the production of hot spots that burn food or cold spots where heating is incomplete. Instances where containers have caught fire have been observed. Consumers obviously become agitated when this happens, and the risk of fire can be a liability for the manufacturer of microwave containers.
A need therefore exists for a method and apparatus for performing a reliable quality check on the susceptors of each microwave container during the container manufacturing process to ensure the integrity of the susceptors and thus the consistency of the cooking experience and safety for ultimate consumers. Such a method and apparatus should be able to operate reliably at throughputs equaling those of the container manufacturing line itself and should automatically reject containers that do not pass the quality check so that they do not find themselves among commercially shipped products. It is to the provision of a method and apparatus that addresses these and other needs that the present invention is primarily directed.